Reduce background FGS start WTF log messages.

In ActivityManagerConstants, add key "fgs_start_allowed_log_sample_rate"
for the "allowed" WTF log sample rate, default sample rate is 25%; add key
"fgs_start_denied_log_sample_rate" for the "denied" WTF log sample rate, default sample rate
is 100%.

In ActiveServices.java, use these two sample rates for the FGS start WTF
log messages.

Bug: 195672687
Test: build and run.
Change-Id: I21cd83552b2988f7e5233094d93f93ba1b6af04a